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The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment was designed to improve the sensitivity of NMR experiments on low-abundant and low magnetogyric ratio nuclei. The net effect is the non-selective polarizarion transfer from protons to X nuclei with the appropriate 1H-X coupling.REQUIREMENTS
Easy implementation on AVANCE spectrometerVERSIONS
The basic INEPT pulse sequence ( 79JACS760 ) consists of the following steps (see INEPT block):EXPERIMENTAL DETAILSModifications of this experiment has been proposed:
- Relaxation period (d1) to achieve a pre-equilibrium state.
- 90º 1H pulse (p1) to create transverse magnetization (Iy).
- An evolution delay optimized to 1/2*J(XH) to achieve antiphase proton magnetization (IxSz). Simultaneous 180º 1H and X pulses are applied at the middle of this period in order to remove 1H chemical shift evolution while heteronuclear couplings are not affected.
- Simultaneous 90º 1H and X pulses to transfer polarization from 1H to X. Antiphase X magnetization is created (IzSx). Note that between the 90º pulses, zz-magnetization has been created (IzSz).
- Acquisition of X nulei during the t2 (acq) period without 1H decoupling.
- Refocused INEPT experiment (see refocused INEPT block) ( 80JMR163-39 , 80JACS6388 , and 81JMR32-44 ): Addition of a refocusing period (delay-180º(1H,X)-delay) in order to obtain in-phase X magnetization. Now, it is possible to apply broadband proton decoupling during acquisition in order to obtain a 1H-decoupled X spectrum.
- The INEPT+ experiment: Addition of a 90º 1H pulse just prior to acquisition of the refocused INEPT experiment in order to remove some dispersive components.
- Compensated INEPT sequences over a range of 1J(XH) values ( 86JMR264-69 and 95JMRA42-115 )
- By incorporating a variable evolution period in the defocusing period, the 2D HETCOR experiment results.
- Nowadays, the INEPT pulse train is largely used as an important building block in multidimensional NMR experiments. See, for instance, the 2D HSQC experiment.
- A reverse INEPT sequence ( 81JMR341-42 and 81JMR8-45 ). Starting from X nuclei, magnetization is transferred to 1H nuclei. The result is the acquisition of the satellites 1H-13C resonances and the suppression by means of phase cycling of the large unwanted central 1H-12C resonance. This block is referred as retro-INEPT building block in many multidimensional experiments.
The INEPT experiments are usually recorded in a fully automated way. Minor changes from a predefined parameter set are required.SPECTRAFor further details on practical implementation of the INEPT experiment on AVANCE spectrometers Tutorial: INEPT experiment
Some enhancement factors for NOE and INEPT experiments with X{1H} pais of nuclei:
X 11B 13C 15N 29Si 57Fe 103Rh 109Ag 119Sn NOE INEPT
The classical INEPT experiment affords a 1H-coupled X spectrum in which X resonances appear as antiphase multiplets with respect 1J(CH). The intensities of the multiplet lines depend of the nuclei being observed. Thus , for instance, a doublet with relative intensity 5:-3 is obtained for a 1H-13C pair compared to the 1:1 doublet obtained in a classical 1H-coupled 13C spectrum. Usually, the phase of the second 90º 1H pulse in the INEPT sequence is inverted (+y and -y) on alternate scans and a relative 4:-4 (or 1:-1) ratio results for a doublet system. Triplets and quartets due to CH2 and CH3 systems are similarly modified according to 1:0:-1 and 1:1:-1:-1 ratios, respectively.RELATED TOPICSIn the refocused version, in-phase X multiplets are obtained. This experiment is usually recorded with broadband proton decoupling, and therefore a 1H-decoupled X spectrum with improved sensitivity is recorded.
Refocused INEPT experiments was used to determine multiplicity of X nuclei, similarly as described for DEPT experiments, by optimizing the refocusing delay.
Similar polarization transfer effects is achieved using the 1D DEPT pulse train, and other sequences have been also used for 13C-multiplicity editing purposes